The invention relates to a mount for supporting a frame and more specifically the invention relates to a mount having a resilient portion and at least one collapsible void provided in the resilient portion, the mount having a first substantially constant stiffness under loading that does not cause the at least one void to be closed and a second substantially constant stiffness under loading which substantially closes the at least one void.
Vehicles, such as trucks which haul cargo have a trailer portion that is supported by a frame. The frame, in turn, is supported in part by a number of spring members such as leaf springs where each spring member extends between longitudinally adjacent ends of parallel wheel axle with the ends of each leaf spring being made integral with the axle ends.
In order to control the vehicle""s vibrational dynamics a bearing or mount is supported on the leaf spring and is connected to the vehicle frame and the spring. The axles are isolated from the frame through the mounts. Transmission of the disturbances from the wheels to the frame is limited by the mounts.
It is most desirable to the vehicle driver to have vehicle mounts that are relatively soft when supported load is at a minimum and are relatively stiff when the supported load is at a maximum. Such a mount would provide the greatest comfort to the vehicle driver and would also improve load stability. Most frequently, prior art mounts for vehicle suspensions have a single stage stiffness that provides the same, single stiffness to the vehicle suspension regardless of the load being supported by the vehicle frame. Such prior art mounts are designed to support large vehicle loads.
Because prior art mounts comprise a single spring rate, the natural frequency of the system varies undesirably as the load supported by the frame is increased and decreased. It is well known to those skilled in the relevant art that the natural frequency of any vibratory system, xcfx89 is equal to the square root of the stiffness of the system spring, k divided by the mass of the system, m. In equation form this relationship may be set forth as xcfx89=k/m where the natural frequency is expressed in cycles per second. Applying this relationship of spring stiffness and mass to current vehicle suspension systems, when the spring rate is constant, the natural frequency of the system decreases as the loading increases, and the natural frequency increases as the magnitude of the load decreases. However, because the single spring rate of prior art mounts is designed to support a loaded frame, the stiffness of the prior art mounts of vehicle support systems is much greater than desired between the minimum and maximum loading conditions.
As indicated hereinabove, in the most desirable vehicle support systems the vehicle suspension is relatively soft when the frame is unloaded and the system stiffness increases as the vehicle is loaded. However such suspension systems with a variable stiffness comprise complicated, expensive devices with a large number of component parts. One type of variable stiffness device for a vehicle suspension comprises multiple mounts arranged in a series relationship. In such devices, the first of the serially arranged mounts is actuated during a first loading range and then the other mounts are actuated as the magnitude of the loading increases and the limits of the actuating loads are exceeded. As the loading increases the first spring element bottoms out, activating the second element of the series, and in combination the elements provide an increased spring rate. In other mounts that provide variable stiffness, the stiffness is modified pneumatically.
Prior art spring or damping elements comprise resilient portions that include one or more cores or voids in order to significantly reduce the stiffness of the spring element in the cored directions. Such spring elements are used to control the motion of the supported device which may be a vehicle engine for example. As the supported device is displaced, the magnitude and direction of such displacement may cause the voids in such prior art spring elements to partially or fully close thereby increasing the stiffness of the spring to limit further displacement of the supported device. A forced displacement of the device may be of such significant magnitude and direction that the voids are closed by the displacement. By closing the voids the displacement of the device reaches its maximum and the displacement is abruptly stopped or xe2x80x9csnubbedxe2x80x9d by the spring element. The stiffness is changed in a non-linear manner, and such changes in stiffness are temporary. Thus in response to a forced displacement such spring elements temporarily and variably increase spring stiffness to limit displacement of a device, and the stored energy in the spring element returns the supported device to the desired location. The prior art spring elements do not provide a first substantially constant stiffness during a first loading range and a second substantially constant stiffness during a second loading range. Additionally, in such prior art spring elements any changes in the stiffness of the spring element are temporary as such devices serve to limit displacement rather than to support static loads.
In summary, such attempts at providing a mount for a vehicle suspension that has an adjustable or dual rate stiffness have resulted in mounts with designs that are complex, expensive and comprise a large number of component parts. Mounts that provide a single constant stiffness are too stiff when the vehicle is unloaded. Other springs serve to variably change spring stiffness in order to limit displacement rather than support static loads.
The foregoing illustrates limitations known to exist in present mounts and vehicle suspension systems. Thus, it is apparent that it would be advantageous to provide an alternative mount suitable for use in a vehicle suspension system where the mount has greater than one constant stiffness and comprises a relatively uncomplicated design. Accordingly, a suitable alternative mount is provided including features more fully disclosed hereinafter.
In one aspect of the present invention this is accomplished by providing a mount comprising a first attachment member a second attachment member spaced from the first attachment member; and at least one resilient member joining the first and second attachment members, each of the at least one resilient members including at least one void, said at least one void being collapsible under loading between a first open void condition where the mount has a first substantially constant stiffness, and a second collapsed condition where each of the voids is substantially closed, the mount having a second substantially constant stiffness when the at least one void is in the second condition.
The mount of the present invention provides a first substantially linear stiffness when the load supported by the frame is of a magnitude between a minimum load value and a predetermined Transition Load. The mount of the present invention provides a second substantially linear stiffness when the load supported by the frame is of a magnitude between the predetermined Transition Load and a maximum load condition. The first and second stiffness values are substantially constant. The second substantially constant stiffness supplied by the mount is significantly greater than the first substantially constant stiffness. When the load supported by the frame is increased to a magnitude at or above the Transition Load, the mount stiffness is abruptly increased to the second substantially constant stiffness and when the load is reduced in magnitude to a magnitude below the Transition Load, the stiffness is abruptly reduced from the second mount stiffness to the first mount stiffness.
The mount of the present invention may comprise a plurality of resilient layers with stiffening members or shims separating each adjacent resilient layer. The shims promote the significant increase in mount stiffness when the mount of the present invention experiences loads at or above the predetermined Transition Loads. When the cores or voids are substantially closed by the Transition loading, the axial thickness of the resilient layer is minimized resulting in a small axial separation between adjacent shims. As a result, the mount produces a significant, abrupt increase in mount stiffness at or above the Transition Load value. Prior art mounts with cores or voids provided in the resilient layer do not include stiffening shims in mount and as a result do not produce the abrupt, significant changes in stiffness that are produced by the mount of the present invention.
Each resilient layer includes at least one void. In one preferred embodiment of the invention each resilient layer includes three voids. They voids of each layer may be aligned in the axial direction and may comprise any suitable cross section. For example, the voids may have a dog-bone shape where the end portions have a greater axial dimension than the portion between the ends, an elliptical shape or a circular shape. The preferred shape for the voids or cores includes end portions that extend outwardly axially a greater distance than the portion between the ends. In this way, as the mount collapses and the cores are closed, stress concentrations in the voids are reduced.
The axial thickness of the resilient layers are varied and the sizes of the voids provided in the resilient layers are varied so that as a load equal to or greater than the Transition Load is applied to the mount, the mount abruptly collapses and assumes an increased stiffness. For example, in the mount of the present invention, the resilient layer made integral with the first attachment member has the maximum layer axial dimension and the layer made integral with the second attachment layer has the minimum layer axial dimension. Additionally, the voids provided in the resilient layer with the maximum axial dimension also have a maximum void area, and the voids provided in the resilient layer with the minimum axial dimension have a minimum void area.